Abstract

Orientation of cell division is critical for plant morphogenesis. This is evident in the formation and function of meristems and for morphogenetic transitions. Mosses undergo such transitions: from two-dimensional tip-growing filaments (protonema) to the generation of three-dimensional leaf-like structures (gametophores). The Defective Kernel 1 (DEK1) protein plays a key role in the perception of and/or response to positional cues that specify the formation and function of the epidermal layer in developing seeds of flowering plants. The moss Physcomitrella patens contains the highly conserved DEK1 gene. Using efficient gene targeting, we generated a precise PpDEK1 deletion (∆dek1), which resulted in normal filamentous growth of protonema. Two distinct mutant phenotypes were observed: an excess of buds on the protonema, and abnormal cell divisions in the emerging buds resulting in developmental arrest and the absence of three-dimensional growth. Overexpression of a complete PpDEK1 cDNA, or the calpain domain of PpDEK1 alone, successfully complements both phenotypes. These results in P. patens demonstrate the morphogenetic importance of the DEK1 protein in the control of oriented cell divisions. As it is not for protonema, it will allow dissection of the structure/function relationships of the different domains of DEK1 using gene targeting in null mutant background.

Δdek1 bud phenotypes are complemented by PpDEK1 cDNA. (a) Physcomitrella patens 14-d-old wild-type (WT) protonemal filament showing a single developing bud (arrow). (b) Fourteen-day-old Δdek1 protonemal filament showing several abortive buds along the filament (arrows). (c) Fourteen-day-old complemented protonemal filament from strain w showing a single developing bud (arrows). (d) Average number of buds in a 15-cell, 14-d-old filament from six different lines. Error bar, ± SD with number of filaments n =100. Each complemented line (w, x, y, z) represents an independent transformation of Δdek1 with cPpDEK1. The significance of bud average difference between strain and the WT was assessed using a t-test: *, P < 1−10; significance of bud average difference between the specific strain and Δdek1 was assessed using a t-test: **, P <1−10; significance of bud average difference between strain and both the WT and Δdek1 was assessed using a t-test; ***, P <1−6). (e) Six-week-old WT gametophores grown on BCDA medium. (f) Isolated mature phyllid from the WT gametophore. (g) Six-week-old complemented gametophores grown on BCDA medium. (h) Isolated mature phyllid from a gametophore, which developed from a complemented line w.